1. Field of the Invention
The present invention is directed to a pulse sequence for a nuclear magnetic resonance tomography apparatus as well as to a nuclear magnetic resonance tomography apparatus operating according to such a pulse sequence.
2. Description of the Prior Art
The invention also relates to techniques that employ pulse sequences wherein, in the steady state, the magnetization vector oscillates between +.alpha./2 and -.alpha./2 given excitation pulses of .+-..alpha.. Examples of such pulse sequences are the SSFP and the FISP pulse sequences.
German PS 44 27 497 discloses a pulse sequence for a magnetic resonance tomography apparatus. In this pulse sequence, a pulse sequence usually referred to as "FISP" (fast imaging with steady precession) is employed. The term "FISP" is a known concept in the field of computed tomography for a specific pulse sequence and is expressly explained, for example, in E. Krestel, "Imaging Systems for Medical Diagnostics", Siemens AG, 1990, pages 544 through 547. According to the pulse sequence disclosed by German PS 44 27 497, such an FISP pulse sequence is modified such that a radio-frequency pulse is emitted in a preparation phase preceding the FISP pulse sequence. This radio-frequency pulse is frequency-selective and is emitted under the influence of a slice selection gradient, so that only a slice of the examination subject is excited. The dephasing caused by the slice selection gradient is in turn canceled by an oppositely directed gradient. The radio-frequency pulse has a flip angle that generates an excursion of the magnetization as occurs in the stationary state of the following pulse sequence. In general, the magnetization vector oscillates between +.alpha./2 and -.alpha./2 given excitation pulses of .+-..alpha., and the radio-frequency pulse must then have a flip angle of .alpha./2 with a angle that is position inverted relative to the following radio-frequency excitation pulse.
In the steady state, as stated, the magnetization vector oscillates between plus .alpha./2 and -.alpha.2 given excitation pulses of .+-..alpha.. For achieving rapid imaging, the spin magnetization represents a problem since this is not yet in the steady state at the beginning of the measurement and leads to signal fluctuations between the echoes, i.e. raw data rows, that produce image artifacts. The method proposed by German PS 44 27 497 solves this problem before the beginning of the actual FISP sequence by placing the magnetization into an approximate condition of the steady state by a preceding RF excitation pulse.
Further, a sequence known as an SSFP pulse sequence (steady state free precession) is known, for example from the aforementioned publication by Krestel, which differs from the FISP sequence essentially in that refocusing gradient pulses are employed in all three directions.
O. Heid et al, "Ultra-Rapid Gradient Echo Imaging", Magnetic Resonance in Medicine, Vol. 33, pages 143 through 149, 1995, disclose a method for rapid imaging on the basis of a gradient echo magnetic resonance technique. An equidistant RF pulse train is thereby applied (see FIG. 1) during a phase of a constant readout field gradient, resulting in a number of k-space paths being produced.
This latter technique is an example of techniques referred to as burst methods which, however, have the disadvantage that the signal that is read out rapidly drops with increasing measuring speed. Given SSFP signals, however, the signal amplitude is preserved given a high repetition rate (echo rate). SSFP techniques, however, have the disadvantage that the obtainable measuring speed is greatly limited by a high number of gradient switching ramps per echo. "Journal of Magnetic Resonance", B 101, pages 106-109, 1993, discloses a pulse sequence wherein the RF excitation pulse train is emitted during a constant magnetic field gradient. The readout of the signal ensues during the same constant magnetic field gradient that was already present during the application of the RF excitation pulse train.
Japanese Application 9-262 219 likewise discloses pulse sequence for nuclear magnetic resonance tomography wherein an RF excitation pulse train is emitted during a positive half of the readout gradient (see FIG. 6), and the signal readout ensues during the switching of a negative half of the readout gradient.
German OS 42 32 883 discloses modulation techniques for radio-frequency pulses usually employed in magnetic resonance.